1. Field of the Invention
The present invention relates to a noise reduction device, in particular, a noise reduction device that reduces a cyclic noise (also known as periodic noise).
2. Description of Related Art
In mobile communication devices, there is a problem that when a noise is mixed into a voice, which is the intended sound, due to surrounding environments, it is very difficult to obtain the voice. In radio devices, in particular, there is a cyclic noise such as a siren that occurs, for example, when firefighters go to the site of a fire or when they fight against the fire at the site. When a radio device is used while a siren is wailing, the siren sound is mixed into a voice and picked up by the radio device, thus causing a problem that the person on the receiving side can hardly catch the voice. Therefore, Japanese Unexamined Patent Application Publications No. S60-033752, No. 2002-258899, No. 2000-293965, No. 2003-58186, No. 2002-367298, and No. H11-232802 disclose techniques for reducing noises.
Japanese Unexamined Patent Application Publication No. S60-033752 discloses a modified version of a speech processing method SPAC (Speech Processing system by use of an Auto correlation function). In the SPAC, a voice, which is the intended signal, can be emphasized by obtaining a short-time auto-correlation at an interval corresponding to one cycle of an input signal and connecting waveforms each corresponding to one cycle of the correlation function. However, although the ability of the SPAC to reduce random noises is high, the effect of the SPAC for cyclic noises is poor because the SPAC has such a characteristic that periodic waveforms are emphasized. Therefore, Japanese Unexamined Patent Application Publication No. S60-033752 makes it possible to reduce the level of cyclic noises as well as random noises by subtracting a waveform obtained by averaging short-time auto-correlation functions in the process in which a voice waveform is synthesized by connecting waveforms each corresponding to one cycle of the correlation function.
However, under the condition where background noises could be mixed into the voices as in the case of mobile communications, there are cases where one cycle cannot be accurately measured by the auto-correlation function. Therefore, in Japanese Unexamined Patent Application Publication No. S60-033752, there is a possibility that discontinuity between frames occurs in the process for synthesizing a voice waveform by connecting waveforms each corresponding to one cycle of the correlation function, thus causing pulse noises. Accordingly, the technique disclosed in Japanese Unexamined Patent Application Publication No. S60-033752 is not suitable for the use in which background noises could be mixed into the voices.
In Japanese Unexamined Patent Application Publication No. 2002-258899, an input signal in which a siren sound is mixed into a voice signal is converted from a time domain to a frequency domain for each frame having a predetermined time length, and the presence/absence of the siren sound is detected from the frequency domain signal. Then, in Japanese Unexamined Patent Application Publication No. 2002-258899, when a siren sound is present, the basic frequency of the siren sound is extracted and the siren sound is suppressed by suppressing a harmonics component(s) several times higher than the basic frequency. Note that in Japanese Unexamined Patent Application Publication No. 2002-258899, as a method for detecting a siren sound, firstly, a point at which the sum total of the spectra of each frequency and its harmonics is maximized is calculated as a basing frequency. Then, a root-mean-square error between the calculated basic frequency and a siren sound fundamental period pattern that is registered in a memory in advance is calculated. When the root-mean-square error is smaller than a predefined threshold, it is determined that a siren sound is present. On the other hand, when the root-mean-square error is larger than the predefined threshold, it is determined that there is no siren sound.
In Japanese Unexamined Patent Application Publication No. 2000-293965, means for sampling an assumed (or expected) mechanical noise signal(s) and storing the sampled noise signal as a pseudo-noise waveform(s) into a memory such as a nonvolatile memory in advance is provided. Then, the pseudo-noise is read from the nonvolatile memory at the noise pitch of a mechanical noise picked up by a microphone and the read pseudo-noise is subtracted from the input signal. By doing so, the noise is reduced.
In Japanese Unexamined Patent Application Publication No. 2003-58186, a siren sound suppression information setting unit detects the presence/absence of a noise to be suppressed from a signal that is converted into a frequency domain. Then, the noise's basic frequency is extracted and supplied to a siren sound suppression unit. Further, in Japanese Unexamined Patent Application Publication No. 2003-58186, this siren sound suppression unit suppresses a siren sound noise based on this information. In this case, the siren sound suppression unit extracts a basic frequency at an interval corresponding to a predetermined frame, so that the memory capacity necessary in a long-term average spectrum amplitude update unit can be reduced. Further, in Japanese Unexamined Patent Application Publication No. 2003-58186, an output of the siren sound suppression unit is supplied to a stationary noise suppression unit and a stationary noise is thereby suppressed.
Each of Japanese Unexamined Patent Application Publications No. 2002-367298 and No. H11-232802 discloses a technique in which a noise is reduce by generating a pseudo-noise signal having a correlation with a noise component mixed into an information signal by using an adaptive filter based on an energy wave generated by using energy generation means and then subtracting the pseudo-noise signal component from the information signal. Further, when the operating mode of an electronic device changes, the noise component of the information signal changes. Therefore, each of Japanese Unexamined Patent Application Publications No. 2002-367298 and No. H11-232802 also discloses a technique in which convergence speed of the noise cancelling is increased by changing a step gain in and near the operating mode transition period of the electronic device.